Commercially popular and successful incremental printing systems primarily encompass inkjet and dry electrographic—i.e. xerographic—machines. (As will be seen, the latter units are only partially incremental.) Inkjet systems in turn focus mainly upon on-demand thermal technology, as well as piezo-driven and variant hot-wax systems.
On-demand thermal inkjet, and other inkjet, techniques have enjoyed a major price advantage over the dry systems—and also a very significant advantage in electrical power consumption (largely due to the energy required to fuse the dry so-called “toner” powder into the printing medium). These advantages obtain primarily in the market for low-volume printing, and for printing of relatively short documents, and for documents that include color images or graphics.
a. Liquid loading, and drying time—On the other hand, in thermal-inkjet technology from the outset it has been necessary to deal with certain intrinsic limitations of the process. First, saturated and satisfyingly rich colors with aqueous inks—particularly to substantially fill the white space between addressable pixel locations—require deposition of large amounts of liquid on the print medium.
This heavy liquid loading must be removed by evaporation (and, for some printing media, absorption) before the printed material can be considered finished. Drying time presents a significant annoyance to users.
Hastening of the drying, however, introduces and aggravates other difficulties such as cockle and other printing-medium deformations, as well as offset and blocking. One popular but only partial solution to these adverse phenomena is the highly elaborated art of printmasking, which divides up all the image inking into two or more deposition intervals or so-called “passes”.
As is well known, however, such tactics greatly prolong the time required to print an image, thereby offsetting much of the benefit of drying-time improvements. The result is to exacerbate the intrinsically lower speed of inkjet systems relative to the xerographic ones—which actually are incremental in only the latent-image formation stage, and substantially holistic at the point of image transfer to the printing medium.
Other techniques for acceleration of drying include heating the inked medium to accelerate evaporation of the water base or carrier. Heating, however, has limitations of its own; and in turn creates other difficulties due to heat-induced deformation of the printing medium.
Glossy stock warps severely in response to heat, and transparencies too can tolerate somewhat less heating than ordinary paper. Accordingly, heating has provided only limited improvement of drying characteristics for these plastic media.
As to paper, the application of heat and ink causes dimensional changes that affect the quality of the image or graphic. Specifically, for certain applications it has been found preferable to precondition the paper by application of heat before contact of the ink; if preheating is not provided, so-called “end-of-page handoff” quality defects occur—such defects take the form of a straight image-discontinuity band formed across the bottom of each page when the page bottom is released.
Preheating, however, causes loss of moisture content and resultant shrinking of the paper fibers. To maintain the paper dimensions under these circumstances the paper is held in tension, and this in turn leads to still other dimensional complications and problems.
Yet all in all the most severe of the backward steps that accompany the benefits of printmodes is the penalty in throughput. This expression of overall printing speed is one of the critical competitive vectors for inkjet printers.
b. Resolution and stability—A second handicap suffered by inkjet systems, particularly in comparison with dry-process machines, is relatively coarser resolution. Although native inkjet resolutions on the order of 48 pixels/mm (1200 dots/inch) are now the state of the art, especially in high-end printer/plotter machines, as a practical matter much of this capability in color reproductions is sacrificed in the rendition process—so that a more-directly comparable figure may be only about 12 pixels/nm, roughly half that of some comparable dry-process printers.
Furthermore use of very fine droplets to fill a pixel grid is sometimes used as a mechanism for mitigating long drying times. Hence the two characteristics—resolution and drying time—are often inherently linked.
In other words, there may not be as many degrees of freedom as may superficially appear. Coarser effective resolution thus takes on a greater significance when considered together with the previously mentioned drying and liquid-loading limitations: these observations suggest a kind of negative synergism between the two.
Another linkage is even more clear—high liquid loading leads directly to so-called “bleed” between adjacent fields of different ink colors, and in the extreme into even the fibers of adjacent unprinted (uninked) printing medium. This is of course particularly noticeable at color boundaries that should be sharp.
The phenomenon of bleed, here introduced as a matter of degraded resolution, can also (or alternatively) be seen as a matter of instability in the deposited image. That is, the image elements placed on the printing medium are failing to remain where placed. This is another fundamental limitation of the inkjet process as conventionally practiced.
c. Gundlach and Parks—In the previously mentioned Gundlach patent document it is suggested that Gundlach's own hot-transfer invention either can print from latent images made just with ions, or can apply the Parks thermal-inkjet method to form an initial image on a conductive drum that has a thin dielectric skin—and print from that initial image. In neither case, however, does Gundlach (or Parks) suggest any strategy for exploiting these ideas to attack the above-discussed drying-time or liquid-loading problems of inkjet printing as such.
d. Korem and Shinkoda—These patents, also mentioned above, relate to stabilization of ink droplets (or color “dots”) on an intermediary surface—for later transfer to paper or other sheet-type printing medium. Stabilization can be promoted by using an intermediary transfer surface that is manufactured with a very small region of material, at each pixel location, that attracts the ink or other colorant substance.
These pixel cells are surrounded by material that repels the same substance, thus creating a dual chemical-affinity differential force for discriminating between desired and undesired colorant positions. As to electrostatic methods, however, Korem and Shinkoda suggest these only for (1) forming or help to form an initial image, as for example a toner image for dry, xerographic systems; or (2) transferring or helping to transfer the colorant from the intermediary surface to the final sheet-type printing medium.
e. Conclusion—Market interest in desktop printers, digital copiers and other types of reproduction equipment continues to increase. The demand for faster and more efficient printing methods has forced designers to push the current implementations to their limits. A fundamental reconfiguration may be required a this point.
In summary, achievement of uniformly excellent inkjet printing continues to be impeded by the above-mentioned problems of drying time and liquid loading—particularly in the mutually exacerbating interaction of these factors with inherently somewhat coarse resolution, or image instability. Thus extremely important aspects of the technology used in the field of the invention remain amenable to useful refinement.